Means for applying a direct current bias voltage to an electrode of a thermionic device



March 13, 1934. D. McDONALD 1,951,251

mums FOR APPLYING A DIRECT CURRENT BIAS VOLTAGE TO AN ELECTRODE OF A THERMIONIC DEVICE Filed April 30, 1951 Fig. I.

ITWVEBTItOT Daniel Tfic Donald H is Attorney- Fatenteci ltlar. l3, 1934 TA'EES N E T E vP'I'ET FFICE Daniel McDonald, Rugby, England, assignor to General Electric Company, a corporation of New York Application April 30, 1931, Serial No. 534,101 In Great Britain May 19, 1930 3 Claims.

The present invention relates to improved means for applying a direct current bias voltage to an electrode of a thermionic device.

It is frequently desired to apply to a grid elec- 5 trode other than the control grid, a fixed bias which is dependent upon the value of the anode potential of the tube. For example, in connection with four-electrode devices, commonly known as screen-grid tubes, in which an extra grid is located between the control grid and the anode, this additional electrode, usually called a screen grid, is commonly maintained at a fixed potential somewhat lower than that of the anode by connecting the screen grid through an impedance of suitable value directly to the positive terminal of the high potential source which is used for providing the operating current for the tube.

While this arrangement is satisfactory for a particular tube, it has been found that if a given tube for which the impedance has been designed is replaced by another tube, the constants of which may not be identical with those of the origie nal tube, the arrangement may not operate satis factorily, since the anode current in tubes of the same type may differ considerably and consequently the anode potential may, in some cases when a tube is used for which the impedance is not particularly designed, fall very nearly to the same potential as that of the screen grid.

An object of the present invention is, in general, to enable a definite potential to be maintained between two electrodes in a thermionic device and particularly to enable the screen grid voltage of a tube to be maintained at a suitable substantially fixed value.

The invention consists in connecting a resistance or impedance, the voltage drop through which is to give the required potential difference between the anode and such other electrode (at its end remote from that at which it is connected to the said electrode), directly to the anode or to a point on the anode circuit which varies proportionally to the anode potential. By such a connection, a change in anode current due to external causes such, for example, as mentioned above, the replacement of a given tube by another tube of similar type, causes a proportionate change to occur on the electrode which is to be maintained at a required potential difference from that of the anode, and enables the operating constants of the circuit to be maintained more nearly at the desired values.

The accompanying drawing illustrates the manner in which the invention is utilized for the application of potential to the screen grid of a four-electrode thermionic device, Fig. 1 being a diagrammatic representation of one form of the invention, and Fig, 2 that of a modification.

In Fig. 1, the anode A of the thermionic tube V is shown as being connected through an impedance Z1 of high direct current resistance to a source of positive potential. It is desirable to maintain the screen grid S at a potential, a definite amount less than that of the anode A. For this purpose I connect the grid S through an impedance Z2, having such a value that under operating conditions the current flow through the impedance is sumcient to produce the required voltage drop across the impedance, directly to the anode A. The screen grid may furthermore be connected through a condenser C to the cathode F of the valve, and in some cases the condenser C may be shunted by a resistance of high value (not shown) With this connection, it will be evident that the potential of the point at which the end of the impedance Z; remote from the grid S is connected will vary in accordance with the anode potential variations and consequently the potential of the screen grid S will also vary in such a manner as to maintain the desire-d potential difference between the anode A and the screen grid S substantially constant.

In the modified form of the invention shown in Fig. 2, the end of the impedance Z2 remote from the screen grid S, instead of being connected directly to the anode A is connectedto a point on the anode impedance Z1 intermediate the ends thereof. In this case, also, the potential at the intermediate point of the impedance Z1 to which the end of the impedance Z2 remote from the screen grid is connected willvary in proportion to the current flowing in the anode circuit, and the potential of the screen grid S will this ad just itself to the anode potential.

What I claim as new and desire to secure by Letters Patent of the United States, is,-

1'. In an amplifying circuit comprising a thermionic device which contains a plurality of electrodes including an anode, means for maintaining a definite potential difference between the anode and another of the electrodes, said means comprising an impedance connected at one end directly to the anode and at the other end directly to the other of said electrodes, the voltage drop across said impedance under operating conditions being equal to the required potential difference.

2. An amplifying circuit including a thermionic device which contains a plurality of electrodes including an anode, a coupling of relatively high eluding an anode;zmeans for maintaining a definite potential difference between the anode and another of the electrodes said means comprising an impedance connected'at one end to a point in the anode circuit between said coupling and the anode, and connected at the other end to the other ofmsaid electrodes, the voltage drop across said impedance under operating conditions being equal to the required potential difference.

DANIEL MCDONALD. 

